[GHC] #8346: Rank 1 type signature still requires RankNTypes

[GHC]

#8346: Rank 1 type signature still requires RankNTypes
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        Reporter:  tinctorius                 |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.6.3
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:  #2605
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Comment (by tinctorius):

 Now, about a fix for this. It can be done quickly, and it can be done
 elegantly.

 * The quick fix: when you see `A -> (forall B. C)`, think `forall B. (A ->
 C)`.

 This specific bug should vanish. However, the problem is bigger:
 {{{
 {-# LANGUAGE ExplicitForAll #-}
 newtype Wrap a r = Wrap (a -> r)
 tuple :: forall a. Wrap a (forall b. b -> (a, b))
 tuple = Wrap $ \x y -> (x, y)
 }}}

 * The elegant fix: when you see `C T1 T2 .. (forall X. TK .. TN)`, and `C`
 is covariant in its `K`-th parameter, think `forall X. C T1 T2 .. TK ..
 TN`.

 This is of course a lot harder. Perhaps the new roles feature can be
 leveraged to carry this information around.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8346#comment:1>
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